The nature of the forces involved in ligand binding by antibodies and substrate binding by enzymes is similar, viz., hydrogen bonding, electrostatic interaction and hydrophobic effect. The energy obtained from enzyme-substrate binding may be visualized to force electronic strain in the substrate and facilitate the formation of a transition state. There is strong evidence for the theory that enzymes bind the transition state of the reaction they catalyze better than the ground state, resulting in a reduced free energy of activation for the reaction (1). This has come to be known as the transition state theory of enzymatic catalysis. Other factors that may facilitate enzymatic catalysis are the proximity and orientation effects--apposition of correctly oriented reactants within the active site of the enzyme would reduce the requirement for a large number of random collisions prior to a productive reactant interaction. In principle, antibodies could catalyze chemical reactions by similar means.
The first report of chemical conversion of a ligand by an antibody appeared in 1980 (2), but the steroid ester hydrolysis by a rabbit polyclonal antiserum described in this report was stoichiometric rather than catalytic. Subsequently, antibodies have been demonstrated to catalyze or facilitate chemical reactions, including acyl transfer (3), pericyclic (4) and redox reactions (5).
It is generally believed that these antibodies obtain their catalytic properties, like enzymes, from their ability to bind the transition state of the ligand better than its ground state. Antibodies with enzymatic activity offer the possibility of specific, high efficiency catalytic chemical conversion of ligands. Many biological mediators are peptides or proteins, including the antigens of pathogenic organisms, hormones, neurotransmitters and tumor specific antigens. It should be possible to utilize the vast repertoire of specificities that the immune system encompasses to catalyze chemical reactions not within the scope of naturally occurring enzymes. The combination of antibody specificity with the catalytic power of enzymes has the potential of generating potent therapeutic agents, e.g., catalytic antibodies capable of specifically hydrolyzing key vital coat proteins, tumor specific proteins, or endogenous proteins involved in disease. Hitherto, antibody mediated cleavage of peptide bonds has not been demonstrated and, thus, the search for antibodies capable of cleaving specific peptide bonds is of considerable interest. Compared to the type of antibody-mediated chemical transformations achieved thus far, the cleavage of peptide bonds is more energy-demanding.
It was also not known that naturally occurring antibodies, i.e., antibodies produced by an animal's immune system to the animal's own cellular component (self-antigen), as opposed to an antigen introduced by immunization, could enhance the rate of a chemical reaction, e.g., the cleavage of a peptide bond. These so-called autoantibodies, which may be found in autoimmune disease are important in a number of therapeutic strategies.